The present invention relates to a method for calibrating a coin checking device comprising at least a sensor.
A coin checking device determines certain features of coins which have been inserted, such as the material, dimensions such as thickness and diameter, light transmitting features, the edge relief and stamping gravure, the weight, the hardness and so on. The materials are typically checked by means of induction coils which magnetic field coacts with the coin material. The coins affect a typical attenuation in the induction sensors, wherein the amount of attenuation indicates the material or the alloy of the coin to be checked. The light transmission of a coin or the relief are mostly checked by optical sensors. A light source illuminates the edge or the surface of the coin and a light electrical receiver receives the pass-through or reflected light to test certain geometrical features of the coin. It is further known to determine the weight or the mass of discs inserted by weighing means or impact sensors. The pulse which is generated by the coin impacting an impact element is characteristical for the mass and thus the weight of the coin. Even the hardness of the coin material may be determined by an impact measurement. The pulse diagram resulting from the impact of a coin on an impact element is an indication for the hardness of the coin.
The known coin checking devices are suited to check a number of different coins. They include a microprocessor and a RAM memory to be loaded with reference values to be compared with sensed values. To accomodate tolerances, it is conventional to provide an upper and a lower reference value for each coin thus defining a so-called admittance band. Before a coin checking device is utilized the reference values have to be loaded in accordance with the coins which should be tested by the device. Theoretically, it is possible to mathematically calculate the reference values, but in practical use this method is not sufficiently accurate. The mechanical and electrical features of a coin checking device are subjected to more or less substantial variations which are mostly due to manufacturing processes. Up to now, it has been considered to be necessary to determine and to programm the reference values specifically for each device.
It is known to conduct a calibrating process of this type by using testing coins. Selected genuine coins having features which are distributed within the acceptance band are inserted into the device to be calibrated. Based on the sensor signals obtained, the reference values are determined and stored. As the testing coins wear in response to time, they have to be replaced by new coins. This turns out to be complicated and difficult. It is also known to use tokens instead of testing coins, such tokens having analog physical features and being made specially for purposes of testing. The method for making tokens, however, is relatively costly.
EP 0 072 189 discloses a method for calibrating coin checking devices using two tokens for each coin set of a certain currency to generate parameter signals. Both the parameter signals characterize the coordinates of a measuring point (angle and length of a point in the vector diagram of electromagnetical behaviour). The parameter signals indicate the mechanical and electrical behaviour of a specific coin checking device during the passage of coins independent from the coin value. The parameter signals are used to calculate calibrating factors to be applied to standard reference values. By means of a proper algorithm the standard reference values are converted to determine reference values to be specific for the device. These reference values are then loaded into the RAM memory of the device. The known method needs less testing coins, but still needs a number of coins. A further drawback is seen in that the reference values are read into the memory during the checking phase. When a coin checking device is being manufactured, it is regularly not known for which currency and thus for which coins it will be used. Accordingly, the device should be calibrated in a late manufacturing process after the particular currency and the particular coins to be tested have become known. Based on manufacturing costs it is preferred that the calibrating process be performed while manufacturing the device.